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+Abstract return types (written `impl Trait` for some trait `Trait`) are only
+allowed as function and inherent impl return types.
+
+Erroneous code example:
+
+```compile_fail,E0562
+fn main() {
+    let count_to_ten: impl Iterator<Item=usize> = 0..10;
+    // error: `impl Trait` not allowed outside of function and inherent method
+    //        return types
+    for i in count_to_ten {
+        println!("{}", i);
+    }
+}
+```
+
+Make sure `impl Trait` only appears in return-type position.
+
+```
+fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
+    0..n
+}
+
+fn main() {
+    for i in count_to_n(10) {  // ok!
+        println!("{}", i);
+    }
+}
+```
+
+See [RFC 1522] for more details.
+
+[RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md